This feature allows you to take a screen of only a part of the display, not all of it. Xiaomi one was of the first companies that introduced the three-finger swipe for taking screenshots. Well, this partial screenshot feature works in a similar fashion.
The feature has been added to MIUI in a May 25 build (20.5.25). In order to enable it, you'll need to navigate to 'Settings'. From there, go to 'Additional Settings', and hit 'Button Shortcuts'. In that menu, you'll see the 'Partial screenshot' option.
Xiaomi added 'Partial Screenshot' feature which can be activated in two ways
You can enable it in order to use gestures to take partial screenshots. You will need to tap the display using three fingers, and hold them in place for a second or two in order for the feature to activate.
Once that happens, you'll be presented with three options, free-form, square, or circle in order to take a screenshot. Those three options are all shown in the video below this paragraph.
At the bottom of the display, you'll see start over, share, edit, and save options. You're getting basically all options that are necessary. Many times, when people are taking a screenshot, they don't need a screenshot of the whole display.
In fact, I'll bet that you don't need a screenshot of the display in the vast majority of situations. If that is the case, then this feature may come in handy.
There is also a second way you can activate the feature, once you enable it. If you don't like the idea of gestures, and don't use them for navigation, you can activate this feature via the home button.
All you have to do is press-and-hold the home button in order to activate it. You'll be presented with the same exact screenshot-taking menu as before.
Xiaomi is not the first smartphone OEM to introduce this feature
Xiaomi is not the first OEM to enable the partial screenshot feature, OPPO did it in ColorOS 7 not long ago. The same goes for Realme, who did it in Realme UI a while back.
Xiaomi's MIUI has come a long way over the years. It is now one of the most compelling iterations of Android. It's not only really smooth, but it's packed with features, and it looks really nice as well.
Do note that MIUI 12 is brand new, and the stable rollout hasn't reached global units yet. It will start at the end of June. If you'd like more information regarding that, click here.